Used F90s are still fetching a high premium, so it would have to be a CPO F10 M5 since OP's monthly budget is $1000.

Both cars share the same engine platform anyway, but I much prefer the DCT on the F10 over the F90's 8-speed automatic transmission. If OP's knees and legs are still good, he could even try to find a 6MT F10 M5, which will be extremely hard to find later down the road.